Twitter, Jaiku and Pownce are microblogging sites. Basically, you post quick updates about what you're doing and people can respond to that. Looks kinda boring in theory, but it's not. And some sites use it to aggregate their RSS feeds to readers (so they create
bot users that post updates when there's a new item in their RSS feed).
Jaiku takes that to the next level. It will allow you to add your Last.fm plays, your Flickr photos, your del.icio.us bookmarks et cetera to your updates.
TwitterJaikuPownce